Botulinum toxin type A is frequently used to treat neuromusculoskeletal impairments by inducing reversible chemical denervation of a muscle. Despite the widespread and increasing utilization of this drug in rehabilitation medicine, little is known about its short and long terms effects on skeletal muscle. The purpose of these experiments is to determine the effects of toxin -induced paralysis on muscle structure, function, and repair. To achieve this goal, three specific aims are proposed. The first aim is to determine the acute and chronic effects of BTX-A treatment on muscle structure and function. Our hypothesis is that BTX-A treatment causes aggressive, muscle fiber degeneration and deranged extracellular matrix resulting in loss of active and passive tension-generating capacity. The second aim is to understand the critical factors guiding on optimal botulinum toxin delivery. Our hypothesis is that optimizing BTX-A delivery (dose, volume, and route) requires an understanding of the architectural complexity, connective tissue matrix, vascularity, and neuromuscular junction distribution with the muscle. The third aim is to understand the mechanism of improved BTX-A efficacy in exercised muscles. We hypothesize therapeutic exercise increases the acute efficacy of BTX-A, reduces acute systemic side effects (by uniformly distributing the toxin and promoting motor neuron uptake), and improves chronic efficacy when redosing is required. The implications of these aims can be measured in four ways. First, the long term ability of skeletal muscle to recover from repeated injections will be understood. Second, the dosing guidelines for botulinum toxin delivery will become muscle specific, improving the efficacy of drug delivery and reducing side-effects. Third, the functionally important passive-tension related changes in response to toxin delivery will be quantified and the mechanism of these changes will be identified. Fourth, the mechanism of improved BTX-A efficiency when delivery is augmented with exercise will be understood. The clinical significance of this work lies in the understanding of the interaction between a pharmacological intervention and muscle function.